icount: don't adjust virtual time backwards after warp

The icount-based QEMU_CLOCK_VIRTUAL runs ahead of the RT clock at times.
When warping, it is possible it is still ahead at the end of the warp,
which causes icount adaptive mode to adjust it backward. This can result
in the machine observing time going backwards.

Prevent this by clamping adaptive adjustment to 0 at minimum.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Message-ID: <20230627061406.241847-1-npiggin@gmail.com>
Cc: qemu-stable@nongnu.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
(cherry picked from commit 67f85346ca9305d9fb3254ceff735ceaadeb0911)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

diff --git a/softmmu/icount.c b/softmmu/icount.c
index 4504433e16872a7b6d5a8b2cdc2de3c73f6de94f..a5cef9c60a77ce7cc6582bb8d88dce4cf60ae0d4 100644 (file)
--- a/softmmu/icount.c
+++ b/softmmu/icount.c
@@ -259,11 +259,16 @@ static void icount_warp_rt(void)
         warp_delta = clock - timers_state.vm_clock_warp_start;
         if (icount_enabled() == 2) {
             /*
-             * In adaptive mode, do not let QEMU_CLOCK_VIRTUAL run too
-             * far ahead of real time.
+             * In adaptive mode, do not let QEMU_CLOCK_VIRTUAL run too far
+             * ahead of real time (it might already be ahead so careful not
+             * to go backwards).
              */
             int64_t cur_icount = icount_get_locked();
             int64_t delta = clock - cur_icount;
+
+            if (delta < 0) {
+                delta = 0;
+            }
             warp_delta = MIN(warp_delta, delta);
         }
         qatomic_set_i64(&timers_state.qemu_icount_bias,